A so-called compound semiconductor device manufactured by using a group III-V compound semiconductor is used as a heterostructure bipolar transistor (HBT) excellent in high-speed operation or various optical devices having a light-emitting function, a light-receiving function, and a light-modulating function and is a component essential to a current optical communication system and a wireless system. In such a compound semiconductor device, as the device is miniaturized to require higher-speed operation, the following problems due to the compound semiconductor easily occur.
In an integrated-circuit technology based on a Si-based material, it is possible to take advantage of a fabrication process technology having a degree of freedom, such as thermal diffusion of impurities, ion implantation, Si oxidation/insulation layer formation, poly-SI deposition, and selective growth. Meanwhile, the compound semiconductor device often has a device configuration based on a mesa structure. Thus, in the compound semiconductor device, there are large restrictions due to problems of the controllability of dimensions associated with mesa processing, the material characteristics, and surface characteristics. Especially, it is difficult to inactivate a surface of the compound semiconductor device (i.e. passivation). The reason why it is difficult to perform passivation is that a level is generated on a surface of the compound semiconductor. The level generated on the surface traps a carrier charge to toughen a control of a potential distribution in a device structure, and, thus, to generate an abnormal forward current associated with a current path of the surface, whereby there occurs a problem that the level becomes a recombination center and increases a dark current.
The problem of the surface characteristics of the compound semiconductor significantly affects when InGaAs with a high electron mobility most suitable for the high-speed operation is used in a base layer and a collector layer of HBT. Further, the problem of the surface characteristics of the compound semiconductor also affects InGaAs and an optical device using a multiple quantum well structure containing InGaAs according to the wavelength (1.5 micron band) used in long-distance optical communication. This is because the bandgap energy of InGaAs is so small as 0.75 eV, and, at the same time, passivation is difficult; therefore, a leakage current in the pn junction tends to increase. The problem of the surface characteristics of the compound semiconductor is common to a heterostructure bipolar transistor, a pin-type photodiode, and so on.
FIG. 6 is a cross-sectional view for explaining a structure of a conventional typical ultrafast HBT 50. In the HBT 50, the structure should be miniaturized to realize the high speed, and usually, a device is constituted by stacking mesa type pn junctions. The HBT 50 is of an npn type in which InGaAs is used in a p-type base layer 506 and an n-type collector layer 503.
In the HBT 50, an InP sub-collector layer 502 electrically separated in an island-shaped manner is disposed on a semi-insulating InP substrate 501, and a base-collector mesa constituted of a low concentration of n-type InGaAs collector layer 503 and a low concentration of p-type InGaAs base layer 506 is disposed on the InP sub-collector layer 502, and an n-type InP emitter layer 507 is disposed on the mesa. The HBT 50 is further provided with an emitter electrode 508, a base electrode 509, and a collector electrode 510. Usually, as in the HBT 50, the p-type base layer 506 and the n-type collector layer 503 are mesas having the same size, and a band diagram in an A-A′ cross section of FIG. 6 is shown in FIG. 7. Thus, an electrical field remains as it is on a mesa side surface of a base-collector junction. In the base-collector junction constituted of InGaAs, passivation of a side surface of the mesa is difficult, and a leakage current in the junction tends to increase. When the forward current increases, the on-voltage of the collector increases, and thus there occurs a problem that operation in a low collector voltage region becomes difficult, and the reliability is impaired due to the instability thereof. When a backward leakage current is large, a portion of a base current flows to the collector side, and therefore, such a problem that a low current operation becomes difficult may occur.
As in a photodiode 60 of FIG. 8 to be described later, a layer equivalent to a non-doped InGaAsP surface cover layer 604 reducing the leakage current may be inserted under the p-type base layer 506. However, this structure increases the size of a collector mesa and may hamper the miniaturization of the device.
FIG. 8 is a cross-sectional view for explaining a structure of a photodiode for communication in which an ultrafast operation is required and, in particular, the photodiode 60 in which a speed of not less than 10 Gb/s is required. In the photodiode 60, a structure of a mesa-type semiconductor layer is often provided on a semi-insulating InP 601 for the purpose of reducing a junction capacity.
The photodiode 60 is constituted of a mesa-processed layer in which an n-type InP contact layer 602, a low concentration of InGaAs light-absorbing layer 603, a low concentration of InGaAsP surface cover layer 604, and a p-type InP contact layer 605 are provided in this order from the lower layer side, and the photodiode 60 further has a p-electrode 606 and an n-electrode 607 required for voltage application.
Unlike the HBT 50 of FIG. 6, in the photodiode 60, a relatively narrow p-type region (p-type InP contact layer 605) is formed in an island-shaped manner on a wide intermediate mesa including the InGaAs light-absorbing layer 603. An active region of a pn junction is a region defined by the p-type InP contact layer 605, and the junction capacity is reduced. FIG. 9(A) is a band diagram in an A-A′ cross section of FIG. 8. As shown in the band diagram, the InGaAs light-absorbing layer 603 is depleted to induce the magnetic field suitable for diode operation.
An upper surface of the InGaAs light-absorbing layer 603 is covered with a non-doped InGaAsP surface cover layer 604 having a larger bandgap preventing exposure of InGaAs. In order to enlarge the upper surface of the InGaAs light-absorbing layer 603, the intermediate mesa is wider compared with a conventional photodiode. By virtue of such a structure, the electric field extending into the side surface of the intermediate mesa is reduced, and the photodiode 60 can suppress an occurrence of a leakage current attributable to a surface of InGaAs.
There has been proposed an inverted photodiode structure in which the polarity of the conductive type of the structure shown in FIG. 8 is switched, a p-type InP contact layer is disposed in the lower portion, and an n-type InP contact layer is disposed in the upper portion (for example, see Patent Document 1). The photodiode disclosed in the Patent Document 1 can suppress the occurrence of a leakage current by similar reasoning.
In the structure of the photodiode 60 of FIG. 8, an electroabsorption modulator having a ridge waveguide is one in which the InGaAs light-absorbing layer 603 is replaced with an optical core layer including InGaAs in an inner structure. As in the case of the photodiode 60, the electroabsorption modulator can suppress the occurrence of a leakage current attributable to an InGaAs surface.